Sputtering targets and method for the preparation thereof

ABSTRACT

A process for the preparation of a sputtering target which comprises sub-stoichiometric titanium dioxide, TiO x , where x is below 2 having an electrical resistivity of less than 0.5 ohm.cm, optionally together with niobium oxide, which process comprises plasma spraying titanium dioxide, TiO 2 , optionally together with niobium oxide, onto a target base in an atmosphere which is oxygen deficient and which does not contain oxygen-containing compounds, the target base being coated with TiO x , which is solidified by cooling under conditions which prevent the sub-stoichiometric titanium dioxide from combining with oxygen.

[0001] The present invention relates to a process for the preparation ofimproved high rate sputtering targets and, in particular, to a processfor the preparation of sputtering targets comprising sub-stoichiometrictitanium dioxide with high electrical conductivity to be used in D.C.sputtering at high power levels.

[0002] Sputtered coatings of various oxides (e.g. silica) and nitrides(e.g. silicon nitride) are used to form optical coatings showinginteresting properties on a number of substrates. Known applicationsinclude low emissivity films on window glasses, cold mirrors onreflectors, enhanced mirrors for photocopiers and antireflectivecoatings on picture glass or TV screens. These coatings are usually madeof stacks of several different layers with different refractive indices,preferably a combination of low and high refractive index, to produceoptical filters. For antireflective coatings it is preferred to combinetwo materials showing the highest and the lowest possible refractiveindices. Such materials are titania and silica. Another advantage ofthese materials is their durability. For low emissivity films on windowglasses it is preferred to combine a silver layer with a high refractiveindex material to dereflect the silver which improves lighttransmission.

[0003] Titanium dioxide coatings have a high refractive index and canthus be used to provide coatings of a high refractive index or toprovide the high refractive index coatings in optical stacks. Theexisting process for producing titanium dioxide coatings comprises usingtitanium metal as the sputtering target and using oxygen as a componentof the plasma gas. The titanium is thus converted to titanium dioxideduring the sputtering process. Although satisfactory coatings oftitanium dioxide can be produced, the rate of deposition is very slowand much slower than coating with zinc oxide and/or tin oxide.

[0004] As a substitute for titanium dioxide it has been suggested to usealternative materials such as niobium oxide. Whilst it is possible tocoat a substrate with niobium oxide using a niobium metal target atslightly higher speeds than the equivalent process using titanium,niobium is very expensive.

[0005] JP-A-07-233469 describes the preparation of a sputtering targetby hot-pressing titanium dioxide powder in a nonoxidizing atmosphere andsintering the compact. The sintered compact comprises TiO_(x) where1<×<2 with a resistivity of 10 ohm.cm which is too high for D.C.sputtering at high power levels. The stability of the sputtering processand the arc rate are both very dependent upon the conductivity of thetarget, particularly at high power levels.

[0006] JP-A-62-161945 describes a method of manufacturing a ceramicsputtering target in which a ceramic material consisting mainly of ZrO₂,TiO₂, SiO₂, Ta₂O₃, Al₂O₃, Fe₂O₃ or a compound of these materials issprayed using a water plasma spray to produce a formed body which may beused as a sputtering target. The sputtering target is used for highfrequency sputtering of non-conductive target materials.

[0007] Accordingly, there is a need for an improved process for theproduction of sputtering targets comprising sub-stoichiometric TiO₂which does not involve the hot-pressing and sintering route ofJP-A-07-233469 and which can be used to produce such targets which havea high enough electrical conductivity to be used as large size targetswith complex shapes at high power levels.

[0008] We have now surprisingly discovered that titanium dioxide can beD.C. sputtered at high power levels from a target comprisingsub-stoichiometric titanium dioxide to provide a coating on a substrateof sub-stoichiometric or stoichiometric titanium dioxide.

[0009] Accordingly, the present invention provides a process for thepreparation of a sputtering target which comprises sub-stoichiometrictitanium dioxide, TiO_(x), where x is below 2 having an electricalresistivity of less than 0.5 ohm.cm, optionally together with niobiumoxide, which process comprises plasma spraying titanium dioxide, TiO₂,optionally together with niobium oxide, onto a target base in anatmosphere which is oxygen deficient and which does not containoxygen-containing compounds, the target base being coated with TiO_(x)which is solidified by cooling under conditions which prevent thesub-stoichiometric titanium dioxide from combining with oxygen.

[0010] Sub-stoichiometric titanium dioxide, TiO_(x), where x is below 2and generally is in the range of from 1.55 to 1.95 is known in the art.When produced according to the process of the present invention theelectrical conductivity will vary, depending upon the stoichiometry, themost preferred form having an electrical resistivity of 0.02 ohm.cm.

[0011] In carrying out the process of the present invention titaniumdioxide, TiO₂ is plasma sprayed onto a target base, such as a backingtube or plate, for example a target base of an electrically conductivematerial, for example stainless steel or titanium metal, aluminium orcopper. The target may be of any type known in the art, for example arotatable target or a flat magnetron target.

[0012] During the plasma spraying process, the action of the plasmaflame on the titanium dioxide causes the titanium dioxide to lose someoxygen atoms from its lattice, preferably from the surface of theparticles. The titanium dioxide is converted into the sub-stoichiometricform, i.e. non-stoichiometric oxygen deficient titania. The primaryplasma gas used for the plasma spraying is preferably argon, withhydrogen as the secondary plasma gas in order to obtain the highesttemperatures of the particles. The titanium dioxide which is subjectedto plasma spraying preferably has a particle size in the range of from 1to 60 micrometers, preferably in the range of from 1 to 20 micrometers.The sub-stoichiometric titanium dioxide which is coated on the targetbase is solidified under conditions which prevent it from regainingoxygen and reconverting to TiO₂. Preferably the target base is watercooled during the plasma spraying in order to quench the titaniumdioxide in the sub-stoichiometric form and to improve the conductivitythereof. It is also important to use a certain amount of hydrogen ornitrogen in the plasma gas in order to produce a high temperature plasmaand to assist in the reduction. Hydrogen is preferred because of itsreducing powers. Preferably particle temperatures of above 2000° C. areused, more preferably above 2500° C.

[0013] In a particular embodiment of the present invention, the titaniumdioxide may be plasma sprayed together with niobium oxide.

[0014] In a further aspect the present invention also provides a processfor the preparation of sub-stoichiometric titanium dioxide, TiO_(x),where x is below 2 having an electrical resistivity of less than 0.1ohm.cm, which process comprises subjecting titanium dioxide to a plasmatreatment in an atmosphere which is oxygen deficient and which does notcontain oxygen-containing compounds. In carrying out this process thetitanium dioxide is preferably sprayed through a plasma flame, forexample a plasma flame using a mixture of argon and hydrogen as theplasma gas. Preferably the plasma flame will operate at a hightemperature to raise the temperature of the particles to above 2000° C.

[0015] The sputtering targets which are produced according to theprocess of the invention have a high electrical conductivity and thusare able to run at high power levels using conventional D.C. powersupplies, without the need for expensive arc diverter systems, or D.C.switching power supplies, or the Twin-Mag System where two targets aresequentially used as anode and cathode with a mid-frequency powersupply, or any special requirements of a gas control system. Using thetargets produced according to the present invention, D.C. sputtering canbe carried out at power levels of up to 100 Kw. The main consequence isthat large target bases, e.g. rotatable 3.5 meters long and 150 mm indiameter can be coated up to a typical coating thickness of 6 mm.

[0016] The targets produced by the process of the present invention donot suffer from significant arcing problems because titanium dioxide hasa higher melting point than titanium metal for which so called “vapourarcing” is a problem due to the lower melting point of the metal. Evenif some arcing does occur for titanium dioxide there is littleaccompanying damage to the target.

[0017] The present invention will be further described with reference tothe following Examples.

EXAMPLE 1

[0018] A rotatable target, water cooled on the inside to 35° C.,comprising a tube of stainless steel of diameter 133 mm and length 800mm was coated to a thickness of from 4 to 10 mm with sub-stoichiometrictitanium dioxide, TiO_(x), where x is below 2 as hereinbefore describedby plasma spraying titanium dioxide (rutile) having a particle size offrom 10 to 40 μm onto the target using argon as the primary plasma gasand hydrogen as the secondary plasma gas. 72 liters (60% argon, 40%hydrogen) were used. The power level was 45 kW (455A, 96V).

EXAMPLE 2

[0019] A commercial white pigment consisting of titanium dioxide in theanatase crystal form was used. This powder is stoichiometric andelectrically insulating. The powder was mechanically agglomerated andcompacted into flakes, ground, sieved (70-100 μm) and sintered at 1300°C. in air. The sintered body was then ground and sieved to a particlesize of 10-40 μm. The particles were yellow stoichiometric,non-conductive, titanium dioxide with a rutile crystalline structure.

[0020] A rotatable target comprising a backing tube of aluminium (2.50 mlong and 133 mm diameter) was prepared by plasma spraying of the aboverutile powder using argon as the primary gas and hydrogen as thesecondary gas. 75 liters (40% argon, 60% hydrogen) were used. The powerlevel was 50 kW (110V, 455A). The plasma spraying was carried out undera nitrogen atmosphere.

[0021] The target was rotated at 100 rpm and the torch translation was2.5 m/min until a coating 4 mm thick was obtained. The inside of thealuminium tube was water cooled to a temperature of 35° C. The coatedtarget had a resistivity of 0.07 ohm.cm. The target was subsequentlytested at power levels of up to 100 kW and worked well in the sputteringequipment without significant arcing. The deposition of titanium dioxidewas six times higher than the rate from a titanium metal target inreactive sputtering.

EXAMPLE 3

[0022] Example 2 was repeated with a low pressure vacuum plasmaoperating at 200 mBar using titanium dioxide in the anatase form havinga particle size in the range of from 1 to 10 μm. Using the low pressureplasma, powders with a smaller particle size can be used.

[0023] On spraying onto a target base under the conditions of Example 2the anatase was converted into a sub-stoichiometric rutile form oftitanium dioxide. The coated target had a resistivity of 0.02 ohm.cm.

EXAMPLE 4

[0024] A mixture of niobium oxide (25 parts by weight) and titaniumdioxide (75 parts by weight) having a particle size of from 0.1 to 2 μmwas agglomerated and compacted, dried and sintered at 1300° C. in air.The sintered body was then ground to a particle size of 10 to 40 μm.

[0025] The powder mixture was then plasma sprayed under the conditionsgiven in Example 2 onto an aluminium backing tube to a coating thicknessof 4 mm. The coated target had an electrical resistivity of 0.5 ohm.cmand thus could be used as a D.C. sputtering target.

1. A process for the preparation of a sputtering target which comprisessub-stoichiometric titanium dioxide, TiO_(x), where x is below 2 havingan electrical resistivity of less than 0.5 ohm.cm, optionally togetherwith niobium oxide, which process comprises plasma spraying titaniumdioxide, TiO₂, optionally together with niobium oxide, onto a targetbase in an atmosphere which is oxygen deficient and which does notcontain oxygen-containing compounds, the target base being coated withTiO_(x) which is solidified by cooling under conditions which preventthe sub-stoichiometric titanium dioxide from combining with oxygen.
 2. Aprocess as claimed in claim 1 wherein the target base is water cooledduring the plasma spraying.
 3. A process as claimed in claim 1 or claim2 wherein the plasma spraying is carried out using argon as the plasmagas and hydrogen as the secondary plasma gas.
 4. A process as claimed inany one of the preceding claims wherein the target base is titanium,stainless steel, aluminium or copper.
 5. A process as claimed in claim 4wherein the target base is a rotatable target or a flat magnetrontarget.
 6. A process as claimed in any one of the preceding claimswherein the titanium dioxide which is plasma sprayed has particle sizein the range of from 1 to 60 micrometers.
 7. A process as claimed in anyone of the preceding claims wherein the titanium dioxide is plasmasprayed together with Nb₂O₃.
 8. A process as claimed in any one of thepreceding claims wherein the sub-stoichiometric titanium dioxide,TiO_(x), has a value of x in the range of from 1.55 to 1.95.
 9. Aprocess as claimed in any one of the preceding claims wherein thesputtering target has an electrical resistivity of about 0.02 ohm.cm.10. A sputtering target comprising sub-stoichiometric titanium dioxide,TiO_(x), where X is below 2 having an electrical resistivity of lessthan 0.5 ohm.cm, optionally together with niobium oxide, which isobtainable by a process which comprises plasma spraying titaniumdioxide, TiO₂, optionally together with niobium oxide, onto a targetbase in an atmosphere which is oxygen deficient and which does notcontain oxygen-containing compounds, the target being coated withTiO_(x) which is solidified by cooling under conditions which preventthe sub-stoichiometric titanium dioxide from combining with oxygen. 11.A sputtering target as claimed in claim 10 which has an electricalresistivity of about 0.02 ohm.cm.
 12. A process for the preparation ofsub-stoichiometric titanium dioxide, TiO_(x), where x is below 2 havingan electrical resistivity of less than 0.1 ohm.cm which processcomprises subjecting titanium dioxide to a plasma treatment in anatmosphere which is oxygen deficient and which does not contain anyoxygen-containing compounds.
 13. A process as claimed in claim 12wherein the titanium dioxide is sprayed through a plasma flame having atemperature of above 2000° C.
 14. A process as claimed in claim 13wherein the plasma flame uses a mixture of hydrogen and argon as theplasma gas.